In this study, the formation of met5 and leu5-enkephalins from high molecular weight enkephalin-like peptides was investigated. A soluble protein fraction prepared from bovine adrenal chromaffin granules was found to produce low molecular weight enkephalin-like peptides (LMW enk-like) when incubated at 37 degrees C. The LMW enk-like peptides include met5- and leu5-enkephalins and other immunoreactive peptides. The productions of LMW met 5-enk like and LMW leu5-enk like were inhibited by soybean trypsin inhibitor and N-alpha-p-tosyl-lysyl-chloro-methylketone respectively. The effectiveness of these trypsin inhibitors suggests that some of the enzymes in the biosynthesis of enkephalins may be trypsin-like in their substrate specificity.